Scr exhaust-gas aftertreatment device and motor vehicle with such an scr exhaust-gas aftertreatment device

ABSTRACT

An exhaust-gas aftertreatment device for an aftertreatment of the exhaust-gas of an internal combustion engine includes an SCR catalytic converter and an SCR particulate filter downstream of the SCR catalytic converter. The SCR catalytic converter has an SCR catalytic coating for a selective reduction of nitrogen oxides in the presence of a reducing agent added to the exhaust-gas in a metered manner. The SCR catalytic converter has a flow-through substrate. The SCR catalytic coating of the SCR catalytic converter is disposed on the flow-through substrate. The SCR particulate filter has an SCR catalytic coating for a selective reduction of nitrogen oxides in the presence of the reducing agent added to the exhaust-gas in a metered manner. The SCR particulate filter has a particulate filter substrate. The SCR catalytic coating of the SCR particulate filter is disposed on the particulate filter substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation, under 35 U.S.C. §120, of copendingInternational Application No. PCT/EP2013/072611, filed Oct. 29, 2013,which designated the United States; this application also claims thepriority, under 35 U.S.C. §119, of German Patent Application No. DE 102012 023 049.4, filed Nov. 26, 2012; the prior applications are herewithincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an exhaust-gas aftertreatment device for anaftertreatment of an exhaust-gas of an internal combustion engine and amotor vehicle with such an exhaust-gas aftertreatment device, whereinthe exhaust-gas aftertreatment device operates in accordance with theprinciple of the selective catalytic reduction.

Internal combustion engines that are constantly or occasionally operatedwith a lean air-fuel mixture produce nitrogen oxides NO_(x) (mainly NO₂and NO), which necessitate NO_(x)-reducing measures. A measure performedby the engine in order to reduce the NO_(x) untreated emission in theexhaust-gas is the exhaust-gas recirculation, in which a part of theexhaust-gas of the internal combustion engine is recirculated into thecombustion air, whereby the combustion temperatures are lowered and thusthe NO_(x) formation is reduced. However, the exhaust-gas recirculationis not always sufficient to comply with legal NO_(x) limit values, whichis why an active exhaust-gas aftertreatment is additionally required,which reduces the NO_(x) final emission by catalytic reduction of NO_(x)to nitrogen N₂. A known NO_(x) exhaust-gas aftertreatment provides forthe use of NO_(x) storage catalytic converters which store nitrogenoxides in the form of nitrates during lean operation (at λ>1) and,during short intervals with a rich exhaust-gas atmosphere (λ<1), desorbthe stored nitrogen oxides and reduce them to nitrogen N₂ in thepresence of reducing agents, which are present in the rich exhaust-gas.

As a further approach for converting nitrogen oxides in exhaust-gases ofcombustion engines that are capable of a lean-burning operation, the useof catalytic converter systems is known which operate in accordance withthe principle of the selective catalytic reduction (SCR selectivecatalytic reduction). These systems include at least an SCR catalyticconverter which selectively converts, in the presence of a reducingagent, usually ammonia NH₃, which is fed to the exhaust-gas, thenitrogen oxides of the exhaust-gas into nitrogen and water. In thiscase, the ammonia can be added in a metered manner from an aqueousammonia solution to the exhaust-gas flow or can be obtained by way ofthermolysis and hydrolysis from a precursor compound, for example ureain the form of an aqueous solution or solid pellets. A more recentapproach to storing ammonia in the vehicle includes NH₃ storagematerials that reversibly bind ammonia as a function of the temperature.In particular metal amine storage systems are known in this context, forexample, MgCl₂, CaCl₂ and SrCl₂, which store ammonia in the form of acomplex compound, in order to then be available for example asMgCl₂(NH₃)_(x), CaCl₂(NH₃)_(x) or SrCl₂(NH₃)_(x). The ammonia can bereleased again from these compounds by the application of heat.

Also known are configurations in which an SCR catalytic converter isdisposed downstream of a particulate filter, in many cases at anunderfloor position of the vehicle. As is the case with all exhaust-gascatalytic converters, SCR catalytic converters too require a specificlight-off temperature in order to provide a sufficient conversionperformance. Depending on the coating of the SCR catalytic converter,exhaust-gas temperatures in the catalytic converter of at least 150° C.are typically necessary. Therefore underfloor SCR catalytic convertersoften require additional heating measures, which result in anundesirable increase in fuel consumption and, consequently, an increasein CO₂ emissions.

In order to minimize the loss of temperature, a positioning of the SCRcatalytic converter close to the engine is also known, in particular byintegrating an SCR catalytic coating in the particulate filter. Such anSCR catalytically coated particulate filter (also called SCR particulatefilter, SCR/PF, or SPF) thus combines the functions of trapping sootparticles and catalytic reduction of nitrogen oxides under selectiveconsumption of the reducing agent. Due to the positioning of the SCRcatalytic converter or, respectively, the SCR/PF close to the engine, arapid heating of this component to its operating temperature isachieved. This allows an early releasing of the reducing agent meteringand thus an improved NO_(x) conversion in the overall driving cycle.However, the temperature gradient across the particulate filtersubstrate compared to that of the flow-through substrate is greaterbecause of the greater substrate length of the filter, which has anadverse effect on the NO_(x) conversion. In individual cases, thesubstrate temperature in the rear area of the particulate filter can beso low that only low conversion rates are achieved in that area.Furthermore, the coating of the particulate filter substrate with theSCR catalytic material, in contrast to the coating of a flow-throughsubstrate (honeycomb body), is limited to smaller amounts of coating, sothat the exhaust-gas back pressure across the particulate filter iswithin acceptable ranges. Thus, the NO_(x) efficiency of the SCRparticulate filter is limited and a downstream SCR catalytic converter,in particular at an underfloor position of the vehicle, is stillrequired. The downstream SCR catalytic converter also serves to preventthe emission of a reducing agent slip of the SCR unit which is close tothe engine.

U.S. Patent Application Publication No. US 2008/0060348 A1 describes anexhaust-gas system having two series-connected SCR catalytic convertersand having a particulate filter disposed between them. By an appropriateselection of the SCR catalytic coatings of the two SCR catalyticconverters, the upstream SCR catalytic converter has a temperaturewindow at lower temperatures than the downstream SCR catalyticconverter. In accordance with an alternative embodiment, U.S. PatentApplication Publication No. US 2008/0060348 A1 proposes to provide theparticulate filter with an SCR catalytic coating and thereby eliminatethe first upstream SCR catalytic converter.

From German Patent Application No. DE 10 2010 026 890 A1, whichcorresponds to U.S. Patent Application Publication No. US 2011/0011068A1, an exhaust-gas system of a diesel engine is known, which has a first“SCR catalytic converter” (HC-SCR catalytic converter) which reducesnitrogen oxides in the presence of hydrocarbons, which are supplied tothe exhaust-gas flow by a fuel metering. Downstream of the HC-SCRcatalytic converter is an oxidation catalytic converter, a second SCRcatalytic converter (NH₃-SCR catalytic converter) and a dieselparticulate filter downstream from that. The SCR catalytic coating ofthe NH₃-SCR catalytic converter is provided on a wall-flow filtersubstrate.

German Patent Application No. DE 10 2010 039 972 A1, corresponding toU.S. Patent Application Publication No. US 2011/0064632 A1, describes aconfiguration which has a first oxidation catalytic converter, anSCR/DPF (selective catalytic reduction/diesel particulate filter)connected downstream thereof, and, downstream of that, an SCR catalyticconverter and, optionally, a second oxidation catalytic converter.

All of the above-mentioned systems have in common that they include anSCR catalytic converter on a particulate filter substrate (SCR/PF) witha downstream-connected SCR catalytic converter on a flow-throughsubstrate.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an exhaust-gasaftertreatment device which allows a further reduction in NO_(x)emissions without having to accept disadvantages in terms of fuelconsumption or, respectively, CO₂ emissions. It is a further object ofthe invention to provide a corresponding motor vehicle having such anexhaust-gas aftertreatment device.

With the foregoing and other objects in view there is provided, inaccordance with the invention, an exhaust-gas aftertreatment device foran aftertreatment of an exhaust-gas of an internal combustion engine,including:

an SCR catalytic converter having an SCR catalytic coating for aselective reduction of nitrogen oxides in a presence of a reducing agentadded to the exhaust-gas in a metered manner, the SCR catalyticconverter having a flow-through substrate, the SCR catalytic coating ofthe SCR catalytic converter being disposed on the flow-throughsubstrate; and

an SCR particulate filter downstream of the SCR catalytic converter, theSCR particulate filter having an SCR catalytic coating for a selectivereduction of nitrogen oxides in a presence of the reducing agent addedto the exhaust-gas in a metered manner, the SCR particulate filterhaving a particulate filter substrate, the SCR catalytic coating of theSCR particulate filter being disposed on the particulate filtersubstrate.

In other words, according to the invention, there is provided anexhaust-gas aftertreatment device for the aftertreatment ofexhaust-gases of an internal combustion engine, wherein the exhaust-gasaftertreatment device includes:

an SCR catalytic converter having an SCR catalytic coating for theselective reduction of nitrogen oxides NO_(x) in the presence of areducing agent added to the exhaust-gas in a metered manner, wherein theSCR catalytic coating is disposed on a flow-through substrate, and

an SCR particulate filter (SCR-PF) having an SCR catalytic coating forthe selective reduction of NO_(x) in the presence of the reducing agentadded to the exhaust-gas in a metered manner, wherein the SCR catalyticcoating is disposed on a particulate filter substrate. The SCRparticulate filter is in this case provided downstream of the SCRcatalytic converter.

In contrast to known SCR exhaust-gas aftertreatment devices, the SCRcatalytic converter in accordance with the invention is thus providedupstream of the SCR particulate filter, and is thus located at aposition closer to the engine. By positioning the SCR catalyticconverter close to the engine (close-coupled position) in front, i.e.upstream, of the SCR particulate filter, the NO_(x) conversionperformance of the SCR catalytic converter is improved due to the lowertemperature gradient in the flow-through substrate of the SCR catalyticconverter. This applies in particular in the case of low exhaust-gastemperatures, for example after a cold start of the internal combustionengine. The configuration according to the invention furthermore allowsfor the reduction or even the absence of heating measures of the SCRcatalytic converter, as a result of which fuel advantages and thus lowerCO₂ emissions are achieved.

The upstream SCR catalytic converter also results in an improvement ofthe contact times between the NO_(x) molecules of the exhaust-gas andthe activity centers of the SCR catalytic coating, which also results inthe improvement of the NO_(x) conversion already at low temperatures.Since the flow-through substrate of the upstream SCR catalyticconverter, when compared to the particulate filter substrate of the SCRparticulate filter, permits a larger amount of SCR coating in relationto the substrate volume, the exhaust-gas back pressure of the entireexhaust-gas aftertreatment device, with the same total amount of the SCRcatalytic coating, is reduced when compared to a single SCR particulatefilter. At the same time lower NO_(x) emissions are achieved with thesame amount of coating when compared to a single SCR particulate filterwithout an upstream SCR catalytic converter. In addition, due to thehigh temperatures during the soot burn-off in the regenerationoperation, the SCR coating of the SCR particulate filter is subject tohigh thermal aging processes which result in a slow deterioration of theNO_(x) conversion. Through the use of the upstream SCR catalyticconverter, the reduction of the NO_(x) activity of the SCR particulatefilter can be compensated.

In the context of the present invention, a flow-through substrate isunderstood to be a catalyst carrier which includes uninterrupted,continuous flow channels from an inflow face side to an outflow faceside, wherein the flow channels are in particular disposed parallel toone another. This can be a ceramic monolith or a metallic catalystcarrier. In contrast, a particulate filter substrate is understood to bea carrier whose flow channels are closed. For example, the particulatefilter substrate can be embodied in the form of a so-called wall-flowfilter, whose parallel flow channels are closed alternately on the inletside or on the outlet side. In this case, a flow channel closed on theoutlet side is disposed adjacent to flow channels closed on the inletside and vice versa. Exhaust-gas which flows into the flow channelsclosed on the outlet side, is thus forced to penetrate through thelateral channel walls, so as to enter the flow channels dosed on theinlet side and thus exit the filter. In this case, particulateconstituents of the exhaust-gas, in particular soot particles, areretained on and in the porous channel walls. Particulate filtersubstrates are usually manufactured from a ceramic material.

In a preferred embodiment of the invention, at least the SCR catalyticconverter is disposed at a position close to the engine. In this way,the light-off temperature of the SCR catalytic converter is quicklyachieved after an engine cold start, and a cooling down of the catalyticconverter during operation is avoided. This permits the elimination ofadditional heating measures for the targeted heat input into thecatalytic converter. In the present case, what is meant by beingpositioned or disposed close to the engine, is a position within theexhaust-gas channel, wherein the position is located upstream of anunderfloor position of a vehicle. In particular, the SCR catalyticconverter dose to the engine is disposed so that a distance between acylinder-side inlet opening of an exhaust-gas manifold of theexhaust-gas aftertreatment device and an inflow face side of the SCRcatalytic converter is at most 100 cm, preferably at most 80 cm. Inspecific embodiments, this distance can even be reduced to values of atmost 70 cm. The distance is in this case measured by the exhaust-gastravel length, i.e. the path length to be traveled by the exhaust-gasbetween the cylinder-side inlet opening of the exhaust-gas manifold andthe inflow face side of the SCR catalytic converter.

In accordance with the invention there is thus provided, in combinationwith an internal combustion engine, an exhaust-gas aftertreatment devicefor an aftertreatment of an exhaust-gas of the internal combustionengine, wherein the exhaust-gas aftertreatment device includes:

an SCR catalytic converter having an SCR catalytic coating for aselective reduction of nitrogen oxides in a presence of a reducing agentadded to the exhaust-gas in a metered manner, the SCR catalyticconverter having a flow-through substrate, the SCR catalytic coating ofthe SCR catalytic converter being disposed on the flow-throughsubstrate;

an SCR particulate filter downstream of the SCR catalytic converter, theSCR particulate filter having an SCR catalytic coating for a selectivereduction of nitrogen oxides in a presence of the reducing agent addedto the exhaust-gas in a metered manner, the SCR particulate filterhaving a particulate filter substrate, the SCR catalytic coating of theSCR particulate filter being disposed on the particulate filtersubstrate; and

at least the SCR catalytic converter being disposed in a position closeto the internal combustion engine.

According to a feature of the invention, the internal combustion enginehas an exhaust-gas manifold with a cylinder-side inlet opening, the SCRcatalytic converter has an inflow face side, a distance between thecylinder-side inlet opening of the exhaust-gas manifold and the inflowface side of the SCR catalytic converter is at most 100 cm, preferablyat most 80 cm.

In a further preferred embodiment of the invention, the downstream SCRparticulate filter is also disposed at a position close to the engine,in which case the distance between the cylinder-side inlet opening ofthe exhaust-gas manifold and the inflow face side of the SCR particulatefilter is at most 120 cm, preferably at most 100 cm.

According to an advantageous embodiment, the SCR catalytic converter hasa smaller volume than the downstream SCR particulate filter. With thismeasure, a very quick light-off of the SCR catalytic converter isachieved after a cold start. In particular, the volume of the SCRcatalytic converter is at most 75%, preferably at most 60% of the volumeof the SCR particulate filter.

Thus, according to another feature of the invention, the SCR catalyticconverter and the SCR particulate filter each have a respective volume,the volume of the SCR catalytic converter is smaller than the volume ofthe SCR particulate filter. In particular the volume of the SCRcatalytic converter is at most 75% of the volume of the SCR particulatefilter. Preferably, the volume of the SCR catalytic converter is at most60% of the volume of the SCR particulate filter.

It is furthermore preferably provided that the SCR catalytic converterhas at least the same or a larger amount of the SCR catalytic coating inrelation to the substrate volume than the SCR particulate filter. Thisembodiment takes account of the fact that flow-through substrates canaccommodate a larger amount of coating per substrate volume than filtersubstrates, without causing unacceptable exhaust-gas back pressuresacross the substrate. Through use of a largest possible amount of SCRcatalytic coating of the SCR catalytic converter, it is possible toadhere to a particularly small catalytic converter volume. Inparticular, the SCR catalytic converter has a greater amount of SCRcatalytic coating than the SCR particulate filter, the amount beinggreater by a factor of at least 1.2, preferably by a factor of at least1.5.

Thus, according to a further feature of the invention, the flow-throughsubstrate and the particulate filter substrate each have a respectivesubstrate volume; and the SCR catalytic converter and the SCRparticulate filter each have a respective amount of the SCR catalyticcoating in relation to the respective substrate volume, the amount ofthe SCR catalytic coating in relation to the substrate volume of theflow-through substrate of the SCR catalytic converter is at least equalto or larger than the amount of the SCR catalytic coating in relation tothe substrate volume of the particulate filter substrate of the SCRparticulate filter. In particular, the amount of the SCR catalyticcoating in relation to the substrate volume of the flow-throughsubstrate of the SCR catalytic converter is larger by a factor of atleast 1.2 than the amount of the SCR catalytic coating in relation tothe substrate volume of the particulate filter substrate of the SCRparticulate filter. Preferably, the amount of the SCR catalytic coatingin relation to the substrate volume of the flow-through substrate of theSCR catalytic converter is larger by a factor of at least 1.5 than theamount of the SCR catalytic coating in relation to the substrate volumeof the particulate filter substrate of the SCR particulate filter.

According to a further advantageous embodiment of the invention, theflow-through substrate of the SCR catalytic converter has a greater cellcount (cell density) than the particulate filter substrate of the SCRparticulate filter. Due to the greater cell count of the SCR catalyticconverter, a large surface area of the cell walls of the flow channelsis achieved, which facilitates the accommodation of a comparativelylarge amount of SCR coating. In particular, the flow-through substratehas a cell count which is greater by a factor of at least 1.1 andpreferably by a factor of at least 1.2 than the cell count of theparticulate filter substrate.

Thus, according to another feature of the invention, the flow-throughsubstrate of the SCR catalytic converter and the particulate filtersubstrate of the SCR particulate filter each have a respective cellcount, the cell count of the flow-through substrate is greater than thecell count of the particulate filter substrate. In particular, the cellcount of the flow-through substrate is greater by a factor of at least1.1 than the cell count of the particulate filter substrate. Preferably,the cell count of the flow-through substrate is greater by a factor ofat least 1.2 than the cell count of the particulate filter substrate.

In particular, the cell count of the flow-through substrate of the SCRcatalytic converter is at least 300 cpsi (cells per square inch),preferably at least 350 cpsi and particularly preferably at least 650cpsi. In contrast, the particulate filter substrate of the SCRparticulate filter has in particular a cell count of at least 250 cpsi,preferably at least 300 cpsi and particularly preferably at least 350cpsi.

Further, the flow-through substrate of the SCR coating has a smallerwall thickness than the particulate filter substrate of the SCRparticulate filter. The wall thickness of the flow-through substrate ispreferably at most 6 mil (1 mil= 1/1000 inch=0.0254 mm), preferably atmost 5.5 mil, more preferably at most 5 mil. In contrast, a preferredwall thickness of the particulate filter substrate of the SCRparticulate filter is at most 30 mil, in particular at most 15 mil andmore preferably at most 13 mil.

The porosity of the particulate filter substrate is in a preferredembodiment at most 65%, in particular at most 61%. The mean pore radiusis preferably ≦25 μm, in particular ≦20 μm.

In accordance with an embodiment of the invention, the SCR catalyticconverter and the SCR particulate filter are disposed in separatehousings connected in series. According to a preferred embodiment,however, the SCR catalytic converter and the SCR particulate filter aredisposed in a common housing, because this results in a furthertemperature advantage as well as a lower exhaust-gas backpressure.

Thus, according to a feature of the invention, the exhaust-gasaftertreatment device includes a common housing, wherein the SCRcatalytic converter and the SCR particulate filter are disposed in thecommon housing.

In a preferred embodiment of the invention, the exhaust-gasaftertreatment device further includes a reducing agent metering device,which is configured to add the reducing agent, or a precursor compoundthereof, in a metered manner to the exhaust-gas upstream of the SCRcatalytic converter. In particular, it is a common metering device forboth, the SCR catalytic converter and the downstream SCR particulatefilter.

According to a further feature of the invention, the exhaust-gasaftertreatment device thus includes a reducing agent metering device,wherein the reducing agent metering device is configured to add thereducing agent or a precursor compound of the reducing agent in ametered manner to the exhaust-gas upstream of the SCR catalyticconverter.

The reducing agent that is added in a metered manner is preferablyammonia NH₃ or a precursor compound thereof, wherein in this case inparticular urea is suitable. The urea may be used in the form of solidurea pellets, but preferably used in the form of an in particular anaqueous urea solution. The urea that is added in a metered manner reactsby way of thermolysis and hydrolysis while releasing NH₃. It is withinthe scope of the invention that the reducing agent ammonia can inprinciple also be stored up through the use of NH₃ storage materialsthat reversibly bind or, respectively, release ammonia as a function ofthe temperature. Corresponding metal amine storages have already beenexplained above.

According to another preferred embodiment of the invention, theexhaust-gas aftertreatment device further includes an oxidationcatalytic converter. The oxidation catalytic converter is preferablydisposed upstream of the SCR catalytic converter. In this manner it isachieved that the NO₂/NO ratio of the exhaust-gas is increased, thusachieving an improved NO_(x) conversion performance of the downstreamSCR components. If furthermore the oxidation catalytic converter isprovided downstream of the reducing agent metering, the oxidationcatalytic converter additionally results in an improved homogenizationof the supplied reducing agent in the exhaust-gas before it enters theSCR catalytic converter.

Thus, according to another feature of the invention, the exhaust-gasaftertreatment device includes an oxidation catalytic converter. Theoxidation catalytic converter is preferably disposed upstream of the SCRcatalytic converter.

The invention further relates to a motor vehicle with an intemalcombustion engine for driving the vehicle and an exhaust-gasaftertreatment device according to the invention.

With the objects of the invention in view there is also provided, amotor vehicle including:

an internal combustion engine;

an exhaust-gas aftertreatment device for an aftertreatment of anexhaust-gas of the internal combustion engine, the exhaust-gasaftertreatment device including an SCR catalytic converter and an SCRparticulate filter;

the SCR catalytic converter having an SCR catalytic coating for aselective reduction of nitrogen oxides in a presence of a reducing agentadded to the exhaust-gas in a metered manner, the SCR catalyticconverter having a flow-through substrate, the SCR catalytic coating ofthe SCR catalytic converter being disposed on the flow-throughsubstrate; and

the SCR particulate filter being disposed downstream of the SCRcatalytic converter, the SCR particulate filter having an SCR catalyticcoating for a selective reduction of nitrogen oxides in a presence ofthe reducing agent added to the exhaust-gas in a metered manner, the SCRparticulate filter having a particulate filter substrate, the SCRcatalytic coating of the SCR particulate filter being disposed on theparticulate filter substrate.

The internal combustion engine is an internal combustion engine that isoperated permanently or at least temporarily in a lean-burn mode, inparticular a diesel engine. The exhaust-gas aftertreatment deviceaccording to the invention can in principle also be advantageously usedfor Otto-cycle engines that are temporarily operated in a lean-burnmode, in particular Otto-cycle engines capable of stratified chargeoperation.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an SCR exhaust-gas aftertreatment device and a motor vehicle withsuch an SCR exhaust-gas aftertreatment device, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic illustration of an exhaust-gas aftertreatmentdevice according to a first embodiment of the invention;

FIG. 2 is a schematic illustration of an exhaust-gas aftertreatmentdevice according to a second embodiment of the invention; and

FIG. 3 is a graph illustrating temporal courses of the NO_(x) untreatedemission of an internal combustion engine and the NO_(x) final emissionin an exhaust-gas aftertreatment device according to the invention witha combination of an SCR particulate filter with an upstream SCRcatalytic converter (dashed lines) as well as a comparison systemwithout an upstream SCR catalytic converter (solid lines).

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is shown a motor vehicle which isonly schematically indicated and is overall designated with referencenumeral 10, wherein the motor vehicle is driven by an internalcombustion engine 12 as a traction source, in particular a dieselengine, and wherein the internal combustion engine is at leasttemporarily operated in a lean-burn mode. The internal combustion engine12 has in this case, for example, four cylinders, however any differentnumber of cylinders is also possible.

The motor vehicle 10 further has an exhaust-gas aftertreatment deviceaccording to the invention, which is overall designated by referencenumeral 14, for the catalytic aftertreatment of an exhaust-gas of theinternal combustion engine 12. The exhaust-gas aftertreatment device 14includes an exhaust-gas manifold 16, which connects the individualcylinder outlets of the cylinders of the internal combustion engine 12to an exhaust-gas channel 18. The exhaust-gas channel 18 has a section20 close to the engine and has an underfloor section 22, which is shownhere in a shortened form and which ends in an exhaust pipe that is notshown here.

Downstream of the exhaust-gas manifold 16, an oxidation catalyticconverter 24 is disposed in the exhaust-gas channel 18. The oxidationcatalytic converter 24 has a flow-through substrate which is coated witha catalytic coating which catalyzes the oxidation of exhaust-gascomponents. In particular, the catalytic coating is suited to convertunburned hydrocarbons HC and carbon monoxide CO into CO₂ and H₂O. Inaddition, the catalytic coating of the oxidation catalytic converter 24is configured to oxidize NO and N₂O to NO₂ in order to increase theNO₂/NO ratio. The catalytic coating of the oxidation catalytic converter24 contains as a catalytic component in particular at least one elementof the platinum group metals Pt, Pd, Rh, Ru, Os, or Ir or a combinationof these, in particular Pt and/or Pd. The catalytic coating furthercontains a washcoat which includes a porous ceramic matrix having alarge specific surface area, for example on the basis of zeolite that isdoped with the catalytic component. The flow-through substrate of theoxidation catalytic converter 24 can be a metallic substrate or aceramic monolith, in particular with a honeycomb-like structure having aplurality of continuous, parallel flow channels. Suitable ceramicmaterials include aluminum oxide, cordierite, mullite and siliconcarbide. Suitable metal substrates are made for example from stainlesssteel or iron-chromium alloys.

Downstream of the oxidation catalytic converter 24, an SCR catalyticconverter 26 is disposed in the section 20 of the exhaust-gas channel 18that is close to the engine. The SCR catalytic converter 26 has, justlike the oxidation catalytic converter 24, a flow-through substrate onmetallic basis or ceramic basis, preferably on ceramic basis. Suitableceramic or metallic materials correspond to those mentioned inconnection with the oxidation catalytic converter. The flow-throughsubstrate of the SCR catalytic converter 26 has a cell count ofpreferably ≧350 cpsi and a wall thickness of ≦5.5 mil. The walls of theparallel and continuous flow channels of the flow-through substrate ofthe SCR catalytic converter 26 are coated with an SCR catalytic coating.These in turn include a washcoat made of a porous ceramic matrix havinga large specific surface area (for example a zeolite on the basis ofaluminum silicate) and catalytic substances disposed thereon in adistributed manner. Suitable SCR catalytic substances include, inparticular base metals such as Fe, Cu, Va, Cr, Mo, W, and combinationsof these. These are deposited on the zeolite and/or the zeolite metalsare partially replaced by ion exchange by the corresponding base metals.

Downstream of the SCR catalytic converter 26 is an SCR particulatefilter 28 which is also disposed in the section 20 of the exhaust-gaschannel 18 that is close to the engine. The SCR particulate filter has aparticulate filter substrate, which is, for example, a wall-flow filter.The particulate filter substrate has parallel flow channels that areclosed alternately on the inlet side and the outlet side. Theparticulate filter substrate is made of a porous ceramic material suchas cordierite, α-aluminum oxide, silicon carbide, silicon nitride,zirconium oxide, mullite, spodumene, aluminum oxide-siliconoxide-magnesium oxide (alumina-silica-magnesia) or zirconium silicate.The cell count of the particulate filter substrate is preferably ≧300cpsi, wherein the cell count is smaller by at least a factor of 1.1 thanthat of the flow-through substrate of the SCR catalytic converter 26.The wall thickness of the particulate filter substrate is preferably atmost 15 mil and has a porosity of ≦61% with a mean pore radius of ≦20μm. The flow channels of the particulate filter substrate are coatedwith an SCR catalytic coating, which in principle can have the samechemical composition as that of the SCR catalytic converter 26. However,the amount of the catalytic coating of the SCR particulate filter 28 inrelation to the substrate volume is less than that of the SCR catalyticconverter 26, in particular by a factor of at least 1.5. The particulatefilter substrate of the SCR particulate filter 28 can be coated with theSCR catalytic coating over the whole area or only in sections, forexample, only in an inlet-side section.

The SCR catalytic converter 26 and the SCR particulate filter 28 aredisposed in a position close to the engine (close-coupled position). Inparticular, the distance D between a cylinder-side inlet opening of theexhaust-gas manifold 16 and an inflow face side of the SCR catalyticconverter 26 is at most 80 cm. Crucial for the measurement of thisdistance D is the actual path length to be covered by the exhaust-gas(the distance D is in this case illustrated in a simplified manner).

In the embodiment shown in FIG. 1, the SCR catalytic converter 26 andthe SCR particulate filter 28 are disposed in a common housing 30, whichhas a conical inlet funnel expanding in the exhaust-gas flow directionand a conically tapering outlet funnel via which funnels it is connectedwith the exhaust-gas channel 18.

The exhaust-gas aftertreatment device 14 further includes a reducingagent metering device 32, with which the reducing agent or a precursorcompound thereof is added to the exhaust-gas in a metered manner. Forexample, the reducing agent is introduced into the exhaust-gas flowthrough the use of a nozzle upstream of the SCR catalytic converter 26.The reducing agent is typically ammonia NH₃, which is added in a meteredmanner in the form of a precursor compound, in particular in the form ofurea. Preferably, the urea, in the form of an aqueous solution, is fedfrom a reservoir, which is not shown, and is added in a metered manner.By way of thermolysis and hydrolysis, the urea is decomposed in the hotexhaust-gas to NH₃ and CO₂.

The metered adding of the reducing agent by the metering device 32 isusually carried out by a control unit which is not illustrated here andwhich controls the device 32 in dependence on an operating point of theengine 12, in particular in dependence on a current NO_(x) concentrationof the exhaust-gas. For the purpose of the control, the exhaust-gasaftertreatment device 14 can also have various exhaust-gas sensors andtemperature sensors, such as NO_(x) sensors upstream and/or downstreamof the SCR components 26/28.

An exhaust-gas aftertreatment device 14 according to a second embodimentof the present invention is shown in FIG. 2. Here, correspondingcomponents are designated with the same reference characters as in FIG.1 and are not discussed in detail again.

In contrast to the embodiment shown in FIG. 1, the SCR catalyticconverter 26 and the SCR particulate filter 28 in FIG. 2 are disposedseparately, each in its own housing 30.

Further variants of embodiments of the exhaust-gas aftertreatment device14, which are not shown here, envisage providing the oxidation catalyticconverter 24 downstream of the reducing agent metering device 32. Inaddition, further exhaust-gas aftertreatment components can be present,for example, at the underfloor position 22 of the exhaust-gas channel18.

FIG. 3 shows cumulative NO_(x) emissions of the exhaust-gas as afunction of time t measured after an engine cold start at time to in astandardized test cycle (here NEDC New European Driving Cycle). In thiscase, the NO_(x) emissions of an exhaust-gas aftertreatment device 14according to the invention corresponding to FIG. 1 (dashed lines 2 and4) and also of a comparison system with an SCR particulate filter 28,but without an upstream SCR catalytic converter 26 (solid lines 1 and 3)were measured. Here, the total amount of the catalytic coating of theSCR particulate filter of the comparison experiment corresponded to thesum of the catalytic coatings of the SCR catalytic converter 26 and theSCR particulate filter 28 of the configuration according to theinvention. The curves 1 and 2 show the respective NO_(x) untreatedemissions (raw emissions), i.e. the NO_(x) emissions in the untreatedexhaust-gas that are generated by the internal combustion engine 12. Thecurves 3 and 4 show the respective NO_(x) final emissions measureddownstream of the SCR particulate filter.

Up to a point in time t₁, the curves of the NO_(x) final emissions (3and 4) correspond to that of the NO_(x) untreated emission 1, 2. Up tothis point in time, the SCR catalytic coatings have in each case not yetreached their light-off temperature, so that no significant NO_(x)conversion takes place. From the point in time t₁ on, the light-offtemperature of the SCR components is reached, so that the NO_(x) finalemissions 3 and 4 are considerably lower than the NO_(x) untreatedemissions 1, 2. In the further course of the curves, also the curves ofthe final emissions 3 and 4 separate from one another, wherein theNO_(x) final emissions of exhaust-gas aftertreatment according to theinvention are significantly below the comparison system. From this it isevident, that despite an identical amount of catalytic material, theexhaust-gas aftertreatment device according to the invention has animproved NO_(x) conversion.

LIST OF REFERENCE CHARACTERS

-   -   10 motor vehicle    -   12 internal combustion engine    -   14 exhaust-gas aftertreatment device    -   16 exhaust-gas manifold    -   18 exhaust-gas channel    -   20 section close to the engine    -   22 underfloor section    -   24 oxidation catalytic converter    -   26 SCR catalytic converter    -   28 SCR particulate filter    -   30 housing    -   32 reducing agent metering device

What is claimed is:
 1. An exhaust-gas aftertreatment device for anaftertreatment of an exhaust-gas of an internal combustion engine,comprising: an SCR catalytic converter having an SCR catalytic coatingfor a selective reduction of nitrogen oxides in a presence of a reducingagent added to the exhaust-gas in a metered manner, said SCR catalyticconverter having a flow-through substrate, said SCR catalytic coating ofsaid SCR catalytic converter being disposed on said flow-throughsubstrate; and an SCR particulate filter downstream of said SCRcatalytic converter, said SCR particulate filter having an SCR catalyticcoating for a selective reduction of nitrogen oxides in a presence ofthe reducing agent added to the exhaust-gas in a metered manner, saidSCR particulate filter having a particulate filter substrate, said SCRcatalytic coating of said SCR particulate filter being disposed on saidparticulate filter substrate.
 2. The exhaust-gas aftertreatment deviceaccording to claim 1, wherein said SCR catalytic converter and said SCRparticulate filter each have a respective volume, said volume of saidSCR catalytic converter is smaller than said volume of said SCRparticulate filter.
 3. The exhaust-gas aftertreatment device accordingto claim 2, wherein said volume of said SCR catalytic converter is atmost 75% of said volume of said SCR particulate filter.
 4. Theexhaust-gas aftertreatment device according to claim 2, wherein saidvolume of said SCR catalytic converter is at most 60% of said volume ofsaid SCR particulate filter.
 5. The exhaust-gas aftertreatment deviceaccording to claim 1, wherein: said flow-through substrate and saidparticulate filter substrate each have a respective substrate volume;and said SCR catalytic converter and said SCR particulate filter eachhave a respective amount of said SCR catalytic coating in relation tothe respective substrate volume, said amount of said SCR catalyticcoating in relation to said substrate volume of said flow-throughsubstrate of said SCR catalytic converter is at least equal to or largerthan said amount of said SCR catalytic coating in relation to saidsubstrate volume of said particulate filter substrate of said SCRparticulate filter.
 6. The exhaust-gas aftertreatment device accordingto claim 5, wherein said amount of said SCR catalytic coating inrelation to said substrate volume of said flow-through substrate of saidSCR catalytic converter is larger by a factor of at least 1.2 than saidamount of said SCR catalytic coating in relation to said substratevolume of said particulate filter substrate of said SCR particulatefilter.
 7. The exhaust-gas aftertreatment device according to claim 5,wherein said amount of said SCR catalytic coating in relation to saidsubstrate volume of said flow-through substrate of said SCR catalyticconverter is larger by a factor of at least 1.5 than said amount of saidSCR catalytic coating in relation to said substrate volume of saidparticulate filter substrate of said SCR particulate filter.
 8. Theexhaust-gas aftertreatment device according to claim 1, wherein saidflow-through substrate of said SCR catalytic converter and saidparticulate filter substrate of said SCR particulate filter each have arespective cell count, said cell count of said flow-through substrate isgreater than said cell count of said particulate filter substrate. 9.The exhaust-gas aftertreatment device according to claim 8, wherein saidcell count of said flow-through substrate is greater by a factor of atleast 1.1 than said cell count of said particulate filter substrate. 10.The exhaust-gas aftertreatment device according to claim 8, wherein saidcell count of said flow-through substrate is greater by a factor of atleast 1.2 than said cell count of said particulate filter substrate. 11.The exhaust-gas aftertreatment device according to claim 1, including acommon housing, said SCR catalytic converter and said SCR particulatefilter being disposed in said common housing.
 12. The exhaust-gasaftertreatment device according to claim 1, including a reducing agentmetering device, said reducing agent metering device being configured toadd one of the reducing agent and a precursor compound of the reducingagent in a metered manner to the exhaust-gas upstream of said SCRcatalytic converter.
 13. The exhaust-gas aftertreatment device accordingto claim 1, including an oxidation catalytic converter.
 14. Theexhaust-gas aftertreatment device according to claim 13, wherein saidoxidation catalytic converter is disposed upstream of said SCR catalyticconverter.
 15. In combination with an internal combustion engine, anexhaust-gas aftertreatment device for an aftertreatment of anexhaust-gas of the internal combustion engine, comprising: an SCRcatalytic converter having an SCR catalytic coating for a selectivereduction of nitrogen oxides in a presence of a reducing agent added tothe exhaust-gas in a metered manner, said SCR catalytic converter havinga flow-through substrate, said SCR catalytic coating of said SCRcatalytic converter being disposed on said flow-through substrate; anSCR particulate filter downstream of said SCR catalytic converter, saidSCR particulate filter having an SCR catalytic coating for a selectivereduction of nitrogen oxides in a presence of the reducing agent addedto the exhaust-gas in a metered manner, said SCR particulate filterhaving a particulate filter substrate, said SCR catalytic coating ofsaid SCR particulate filter being disposed on said particulate filtersubstrate; and at least said SCR catalytic converter being disposed in aposition close to the internal combustion engine.
 16. The exhaust-gasaftertreatment device according to claim 15, wherein the internalcombustion engine has an exhaust-gas manifold with a cylinder-side inletopening, said SCR catalytic converter has an inflow face side, adistance between the cylinder-side inlet opening of the exhaust-gasmanifold and said inflow face side of said SCR catalytic converter is atmost 100 cm.
 17. The exhaust-gas aftertreatment device according toclaim 16, wherein the distance between the cylinder-side inlet openingof the exhaust-gas manifold and said inflow face side of said SCRcatalytic converter is at most 80 cm.
 18. A motor vehicle comprising: aninternal combustion engine; an exhaust-gas aftertreatment device for anaftertreatment of an exhaust-gas of said internal combustion engine,said exhaust-gas aftertreatment device including an SCR catalyticconverter and an SCR particulate filter; said SCR catalytic converterhaving an SCR catalytic coating for a selective reduction of nitrogenoxides in a presence of a reducing agent added to the exhaust-gas in ametered manner, said SCR catalytic converter having a flow-throughsubstrate, said SCR catalytic coating of said SCR catalytic converterbeing disposed on said flow-through substrate; and said SCR particulatefilter being disposed downstream of said SCR catalytic converter, saidSCR particulate filter having an SCR catalytic coating for a selectivereduction of nitrogen oxides in a presence of the reducing agent addedto the exhaust-gas in a metered manner, said SCR particulate filterhaving a particulate filter substrate, said SCR catalytic coating ofsaid SCR particulate filter being disposed on said particulate filtersubstrate.